As a wheel that can be used in omni-directional propulsion systems such as vehicles, transporters and robots, it is known to fit a plurality of free rollers on an annular core member such that each free roller may be freely rotatable around the tangential direction of the annular core member at the corresponding position. See JP2010-247640A and WO2010/64408A, for instance. Such a wheel is known as an omni-wheel, and allows the vehicle fitted with such a wheel to travel not only in the fore and aft direction owing to the rotation of the annular core member around the center line thereof but also in the lateral direction owing to the rotation of the individual free rollers fitted on the annular core member around the respective tangential lines. By combining these two traveling modes, the vehicle is allowed to travel in any oblique direction as desired.
The annular core members of conventional omni-wheels were known to be costly and heavy because they were made by machining or casting metallic members. Also, to allow the free rollers to be fitted on the annular core member, the annular core member has to be made of two pieces or two halves which are joined to each other once all the free rollers are in place. Various proposals have been made for joining the two pieces together, but have suffered from the problems of high complexity and high cost. Furthermore, in order to ensure the necessary mechanical strength, the annular core member is required to have a significant thickness, and hence a significant weight.
Therefore, there is a need for a light, economical and strong annular core member that can be advantageously used for building an omni-wheel.